Project information

Project financing:

Bayerisches Staatsministerium für Wissenschaft, Forschung und Kunst

Abstract

In times of climate change and increasing carbon dioxide concentrations, three questions arise for ecosystem sciences: At first, which ecosystems can contribute to mitigate those processes? Secondly, how will ecosystems react on the changing conditions? And finally, is the performance of our commonly applied research methods adequate under those complex and continuously changing environmental conditions? This thesis is integrated in the joint research project FORKAST which investigates those questions. Therefore, micrometeorological methods (Eddy-Covariance – EC, Relaxed Eddy Accumulation – REA), chamber measurements and isotopic methods are applied. First, the investigated area, an extensively managed grassland, was defined as a net carbon sink (–91 g C m–2 a–1) in 2010. Spring droughts on the site suggested comparing carbon allocation into shoots, roots, soil and respiration fluxes on drought and normal plots in a stable isotope pulse labeling experiment. An increase of carbon allocation by 6.2% to below ground pools as soil and roots and a reduction of shoot respiration by 8.5% due to spring drought were found. Gaining absolute values of carbon allocation, the relative portion, provided by pulse labeling and tracing, was set off the absolute carbon input into the ecosystem, obtained by EC-measurements of the net ecosystem carbon exchange in combination with model based partitioning of that into underlying assimilation and respiration flux. With the absolute carbon input of –7.1 g C m–2 d–1 and the relative allocation of the labeling, into fluxes of 2.5, 0.8, 0.5, 2.3 and 1.0 g C m–2 d–1 into shoots, roots, soil, shoot respiration and CO2 efflux could be determined.
Flux partitioning was also achieved by an isotopic approach, based on isoflux measurements with the REA-technique. However, certain restrictions for applying relaxed eddy accumulation on managed grassland were found: Scalar similarity could not be guaranteed directly after the management. It is suggested to wait at least 22 days in summer and 12 days in autumn after the management.
The chamber method was applied to validate the assimilation flux, provided by the common flux partitioning model. This was done during the day at time of turbulent atmospheric conditions. In a comparison experiment between the chamber and eddy-covariance a good agreement was found at that time. In the late afternoon and during night, the chamber could not reproduce present atmospheric conditions, as, for example, increasing stable stratification due to the oasis effect or coherent structures. This resulted in smaller chamber CO2 source fluxes of 26% during night and larger chamber CO2 sink fluxes of 14% during day. The chamber technique is important for small scale measurements (especially in treatment experiments). Thus, it is important to know the reasons for those differences to eddy-covariance.